Electronic circuits referred to as H-bridges are often used drive, i.e. provide power to, electric motors. An H-bridge can act as a series of switches that, when closed, provide a path for current flowing through the motor. A four-switch or “full” H-bridge can be used to turn a motor off, turn a motor on in a forward direction, and turn a motor on in a reverse direction.
A two-switch or “half” H-bridge has two switches that can be used to provide power to a load. Half H-bridges are sometimes used in motor driver circuits, switching amplifier circuits, switching power supply circuits, and the like.
The switches in Hi-bridge circuits are often implemented by electronic switches such as transistors, i.e. field effect transistors (FETs) or BJT transistors. As is known, in order to turn a FET on or off, a voltage must be applied to the transistor's gate. In the case of a BJT, a current supplied to the transistor's base is used to turn the transistor on and off. However, in some H-bridge designs it may be difficult to drive the gate of the transistor hard enough under low voltage conditions to adequately turn the FET on or off. In other H-bridge designs, if the gate of the transistor is driven too hard under high voltage conditions, and a gate-source voltage (Vgs) becomes too great, the FET can become damaged. As is known in the art, FETs typically have source, drain, and gate terminals used in schematic circuit drawings.
In many applications, an H-bridge must be able to operate under both high and low voltage conditions. For example, an H-bridge that drives a pump motor in an automobile may be subject to large swings in the voltage supplied by the battery or alternator. These swings can be caused by the engine starting up, the engine running at varying speeds, electric window or wiper motors turning on and off etc. For these reasons, it would be beneficial to provide circuitry that can drive the switches of an H-bridge under high, low, and normal voltage conditions.